Given that the Gateway Approach is carried out by a researcher who has an 'inside' position, this must be a problem. Mears explains:

In any research project, whatever the methodology, the potential for subjectivity exists, and in interview-based research, a personal incination to a particular point of view could introduce a bias that would make the research of dubious worth.

How can this be countered? Mears suggests:

Instead of asserting that it is possible to achieve a distanced objectivity, it seems more productive to proceed from a position of reflection, candour and disclosure.

Also refered to as positionality, it is important for the researcher to disclose their position so that readers of the research can calibrate or allow for the original researcher's point of view.

Yes. And it also makes a difference if the researcher is coming to a point of view or whether he/she starts with that viewpoint and is endeavouring to establish it.

I'm not so sure that there is much of a difference in either case between the insider or the outsider. The latter may try to say he is less biased by virtue of being an outsider, but that doesn't appear to me to hold much water.